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    SubjectRe: [PATCH, RFC] v4 scalable classic RCU implementation
    On Sat, Sep 06, 2008 at 06:37:38PM +0200, Manfred Spraul wrote:
    > Hi Paul,
    >
    > Paul E. McKenney wrote:
    >> o The rcu_pending() and rcu_needs_cpu() primitives are now
    >> much more aggressive about permitting CPUs to enter dynticks
    >> idle mode. Only CPUs that have RCU callbacks are kept out
    >> of dynticks idle mode.
    >>
    >
    > I've noticed that right now rcu_enter_nohz() can be nested within
    > rcu_irq_enter():
    > irq_exit() first calls tick_nohz_stop_sched_tick(), then rcu_irq_exit().
    > And tick_nohz_stop_sched_tick() can switch into nohz mode.
    >
    > Is that intentional? Does rcupreempt support that? It broke my rcustate
    > code on x86-64.
    >
    > I would prefer if something like the attached patch is applied. What do you
    > think?
    > Do you need the patch as well?

    Good question -- when I tried splitting irqs from NMIs, things broke
    badly, and this might well explain it. Thank you very much for the
    hint!!!

    Thanx, Paul

    > --
    > Manfred

    > diff --git a/kernel/softirq.c b/kernel/softirq.c
    > index ba20a90..cca5a83 100644
    > --- a/kernel/softirq.c
    > +++ b/kernel/softirq.c
    > @@ -284,10 +284,10 @@ void irq_exit(void)
    > invoke_softirq();
    >
    > #ifdef CONFIG_NO_HZ
    > + rcu_irq_exit(0);
    > /* Make sure that timer wheel updates are propagated */
    > if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
    > tick_nohz_stop_sched_tick(0);
    > - rcu_irq_exit(0);
    > #endif
    > preempt_enable_no_resched();
    > }



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